1. Field of the Invention
This invention relates to heat exchangers, and more particularly to heat exchangers for cooling engines, generators, gear boxes and other heat generating sources in industrial apparatuses having fluid cooled heat sources, such as marine vessels. The invention more particularly relates to open heat exchangers (where heat transfer tubes are exposed to the ambient cooling or heating fluid, rather than being in a shell to shell container holding the cooling or heating fluid) used for cooling heat sources, where the heat exchangers are efficient, and thus have lower weight and volume compared to other heat exchangers known in the art. Alternatively, the heat exchanger according to the invention could be used as heater, wherein relatively cool fluid absorbs heat through the heat transfer tubes.
2. Description of the Prior Art
Heat generating sources in industrial applications such as marine vessels are often cooled by water, other fluids or water mixed with other fluids. For example, in marine vessels used in fresh water and/or salt water, the cooling fluid or coolant flows through the engine or other heat generating source where the coolant picks up heat, and then flows to another part of the plumbing circuit. The heat must be transferred from the coolant to the ambient surroundings, such as the body of water in which the vessel is located. For small engines, such as outboard motors for small boats, ambient water pumped through the engine is a sufficient coolant. However, as the vessel power demand gets larger, ambient water pumped through the engine may continue to provide good cooling of the engine, but also serves as a source of significant contamination damage to the engine. If raw, ambient water were used to cool the engine, the ambient water would carry debris and, particularly if it is salt water, corrosive chemicals to the engine. Therefore, there have been developed various apparatuses for cooling engines and other heat sources. One apparatus for cooling the engine of a vessel is channel steel, which is basically a large quantity of shaped steel which is welded to the bottom of the hull of a vessel for conveying engine coolant and transferring heat from the coolant to the ambient water. Channel steel has severe limitations: it is very inefficient, requiring a large amount of steel in order to obtain the required cooling effect; it is very expensive to attach to a vessel, since it must be welded to the hullxe2x80x94a very labor intensive operation; since channel steel is very heavy, the engine must be large enough to carry the channel steel, rendering both the initial equipment costs and the operating costs very high; the larger, more powerful engines of today are required to carry added channel steel for their cooling capacity with only a relatively small amount of room on the hull to carry it; the payload capacity is decreased; the large amount of channel steel is expensive; and finally, channel steel is inadequate for the present and future demands for cooling modem day, marine vessels. Even though channel steel is the most widely used heat exchanger for vessels, segments of the marine industry are abandoning channel steel and using smaller keel coolers for new construction to overcome the limitations cited earlier.
A keel cooler was developed in the 1940""s and is described in U.S. Pat. No. 2,382,218 (Femstrum). The Femstrum patent describes a heat exchanger for attachment to a marine hull structure which is composed of a pair of spaced headers secured to the hull, and a plurality of heat conduction tubes, each of whose cross-section is rectangular, which extend between the headers. Cylindrical plumbing through the hull connects the headers to coolant flow lines extending from the engine or other heat source. Hot coolant leaves the engine, and runs into a heat exchanger header located beneath the water level (the water level refers to the water level preferably below the aerated water, i.e. below the level where foam and bubbles occur), either beneath the hull or on at least one of the lower sides of the hull. The coolant then flows through the respective rectangular heat conduction tubes and goes to the opposite header, from which the cooled coolant returns to the engine. The headers and the heat conduction tubes are disposed in the ambient water, and heat transferred from the coolant, travels through the walls of the heat conduction tubes and the headers, and into the ambient water. The rectangular tubes connecting the two headers are spaced fairly close to each other, to create a large heat flow surface area, while maintaining a relatively compact size and shape. Frequently, these keel coolers are disposed in recesses on the bottom of the hull of a vessel, and sometimes are mounted on the side of the vessel, but in all cases below the water line.
The foregoing keel cooler is referred to as a one-piece keel cooler, since it is an integral unit with its major components welded or brazed in place. The one-piece keel cooler is generally installed and removed in its entirety.
There are various varieties of one-piece keel coolers. Sometimes the keel cooler is a multiple-pass keel cooler where the headers and heat conduction tubes are arranged to allow at least one 180xc2x0 change in the direction of flow, and the inlet and outlet ports may be located in the same header.
Even though the foregoing heat exchangers with the rectangular heat conduction tubes have enjoyed wide-spread use since their introduction over fifty years ago, they have shortcomings which are corrected by the present invention.
The rectangular heat exchangers of the prior art have the outward shape of a rectangular parallelepiped having headers at their opposite ends. These headers have opposing end walls which are perpendicular to the hull of the vessel and parallel to each other, and act as a barrier to ambient water flow relative to the keel cooler as the vessel with the heat exchanger travels through the water. The perpendicular header walls are responsible for the creation of dead spots (lack of ambient water flow) on the heat exchanger surfaces, which largely reduce the amount of heat transfer occurring at the dead spots. In addition, the perpendicular walls diminish the flow of ambient water between the heat conduction tubes, which reduces or diminishes the amount of heat which can be transferred between the coolant in the tubes and the ambient water.
The ability of a heat exchanger to efficiently transfer heat from a coolant flowing through heat conduction tubes depends, in part, on the volume of coolant which flows through the tubes and its distribution across the parallel set(s) of tubes, and on whether the coolant flow is turbulent or laminar. The volume flow of coolant per tube therefore impacts heat transfer efficiency and pressure drop across the heat exchanger. In the present heat exchanger with rectangular tubes, the ends or extensions of the outermost rectangular tubes form exterior walls of the respective headers. Coolant flowing through the heat exchanger, has limited access to the outermost tubes as determined from data obtained by the present inventors. At the present time, the outermost tubes have a solid outer wall, and a parallel inner wall. In order for coolant to flow into the outermost rectangular tubes, orifices, most often circular in shape, are cut through the inner wall of each of the outer tubes for passing coolant into and out of the outer tubes. The inlet/outlet orifices of the exterior tubes are presently disposed centrally in a vertical direction and endwardly of the respective headers of the keel coolers. However, an analysis of the flow of coolant through the foregoing keel cooler shows that there is a larger amount of coolant per tube flowing through the more central tubes, and much less coolant per tube through the outermost tubes. A graph of the flow through the tubes has a general bell-shaped configuration, with the amount of flow decreasing from the central portion of the tube array. The result is that heat transfer is lower for the outermost tubes, and the overall heat transfer for the keel cooler is also relatively lower, and therefore, the pressure drop across the keel cooler is higher than desired.
The flow of coolant through the respective orifices into the outermost rectangular tubes was found to be inefficient, causing insufficient heat transfer in the outermost tubes. It was found that this occurred because the orifices were located higher and further towards the ends of the respective headers than is required for optimal flow. It has been found by the inventors that enlarging the orifice size and moving it closer to the natural flow path of the coolant flowing through the headers, i.e. its optimal path of flow, coupled with the modification to the design of the header as discussed below, further increased the flow to the outer tubes and made the flow through all of the tubes more uniform, reducing the pressure drop across the cooler while increasing the heat transfer.
The current keel cooler with rectangular heat conduction tubes has an anode and a drain plug or plugs located on the bottom portion of the respective headers, which increases the overall height of the header and which may render these devices subject to potential damage from debris in the water and underwater structures. In order to reduce the likelihood of damage, shrouds have been provided to protect the keel coolers against damage. In addition, the anode(s), and the drain plug(s), by projecting into the ambient water, impede the relative flow of the ambient water as the vessel moves therethrough which increases drag. As explained below, the location of the anode(s) and drain plug(s) so as to minimize the increase height of the header and the keel cooler, reduces the foregoing problems.
As discussed below, the beveled header, and the relocation of the anode assemblies and drain plugs, also contribute to the increase of the overall heat transfer efficiency of the keel cooler according to the invention, since the ambient water is caused to flow towards and between the respective heat conduction tubes, rendering the heat transfer substantially higher than in the keel cooler presently being used. This increase in heat transfer is due at least in part to the increase in turbulence in the flow of ambient water across the forward header and along and between the coolant flow tubes.
One of the important aspects of keel coolers for vessels is the requirement that they take up as small an area on the vessel as possible, while fulfilling or exceeding their heat exchange requirement with minimized pressure drops in coolant flow. The area on the vessel hull which is used to accommodate a keel cooler is referred to in the art as the footprint. In general, keel coolers with the smallest footprint and least internal pressure drops are desirable. One of the reasons that the keel cooler described above with the rectangular heat conduction tubes has become so popular, is because of the small footprint it requires when compared with other keel coolers. However, keel coolers according to the design of rectangular tubed keel coolers presently being used have been found by the present inventors to be larger than necessary both in terms of size and the related internal pressure drop. By the incorporation of the various aspects of the present invention described above (and in further detail below), keel coolers having smaller footprints and lower internal pressure drops are possible. These are major advantages of the present invention.
Some of the shortcomings of heat exchangers with rectangular heat conduction tubes presently being used relate to the imbalance in the coolant flow among the parallel tubes, in particular in keel coolers which lead to both excessive pressure drops and inferior heat transfer which can be improved according to the present invention. The unequal distribution of coolant flow through the heat conduction tubes in present rectangular tube systems has led to inferior heat transfer in the systems. In order to attend to this inferior heat transfer, the designers of the present keel coolers on the market have been compelled to enlarge or oversize the keel cooler which also may increase the footprint, through additional tube surface area, to overcome the poor coolant distribution and inferior heat transfer in the system. This has resulted in the present one piece keel coolers which are unnecessarily oversized when compared with the invention described below. In some instances, the invention described below would result in fewer keel coolers in cooling circuits which require multiple keel coolers.
The unequal distribution of coolant flow through the heat conduction tubes in present rectangular tube systems also results in higher internal pressure drops in the systems. This higher pressure drop is another reason that the prior art requires oversized heat exchangers. Excessive oversizing compensates for poor heat transfer efficiency and excessive pressure drops, but this requires added costs and a larger footprint.
When multiple pass (usually two pass) keel coolers are specified for the present state of the art, an even greater differential size is required when compared with the present invention, as described below.
It is an object of the present invention to provide a heat exchanger for fluid cooled heat sources which is smaller than corresponding heat exchangers having the same heat exchange capability.
Another object of the present invention is to provide an improved heat exchanger for industrial applications which is more efficient than heat exchangers presently known and used.
It is yet another object of the present invention to provide an improved one-piece heat exchanger for vessels which is more efficient in heat transfer than presently known one-piece heat exchangers.
It is an additional object to produce a one-piece heat exchanger and headers thereof which generally equalizes the flow of coolant through each of the tubes of the keel cooler.
A further object is to provide an improved one-piece heat exchanger which reduces the pressure drop of coolant flowing therethrough.
A further object of the present invention is to provide an improved one-piece heat exchanger having heat conduction tubes which are rectangular in cross-section having a length which is reduced in size from the current heat exchangers due to improved coolant flow distribution inside the heat exchanger and enhanced ambient water flow across the keel cooler.
Another object is to provide an improved one-piece heat exchanger having a reduced size from present one-piece heat exchangers of comparable heat transfer capability, by reducing the length of the heat transfer tubes, the number of tubes and/or the size of the tubes.
A still further object of the present invention is to provide a new one-piece heat exchanger having rectangular shaped heat conduction tubes which has enhanced durability compared to keel coolers presently on the market.
A related object of the invention is to provide an improved heat exchanger and headers thereof which is capable of deflecting debris more readily, and for presenting a smaller target to debris in the ambient water.
It is another object to provide a keel cooler and header thereof which projects into the water from the hull by a lesser amount than the corresponding one-piece keel coolers and headers thereof.
Another object of the present invention is to provide an improved one-piece keel cooler which is easier to install on vessels than corresponding keel coolers presently on the market.
It is still another object of the invention to provide a one-piece heat exchanger having a reduced pressure drop and a more uniform distribution of coolant flowing therethrough than heat exchangers presently on the market, for increasing the amount of coolant flowing through the heat exchanger to improve its capacity to transfer heat.
Yet a further object of the present invention is to provide a one-piece heat exchanger and a header having a lower weight, and therefore lower cost, than corresponding one-piece heat exchangers presently in use.
Another object of the present invention is to provide a one-piece heat exchanger and headers thereof having rectangular heat conduction tubes having a lower pressure drop in coolant flowing through the heat exchanger than corresponding heat exchangers presently known.
Another object of the present invention is the provision of a one-piece heat exchanger for a vessel, for use as a retrofit for previously installed one-piece heat exchangers which will surpass the overall heat transfer performance and provide lower pressure drops than the prior units without requiring additional plumbing, or requiring additional space requirements, to accommodate a greater heat output.
It is another object of the invention to provide an improved header for a one-piece heat exchanger having rectangular coolant flow tubes.
Another object is to provide an improved header for a one-piece heat exchanger with rectangular coolant flow tubes which reduces the dead spots which have heretofore reduced the heat transfer capabilities of one-piece heat exchangers, the dead spots reducing the flow of ambient water around and between the coolant flow tubes.
A further object of the invention is to provide an improved header for a one-piece keel cooler with rectangular coolant flow tubes, by reducing the likelihood of damage to the header from striking debris and underwater objects which could damage the keel cooler.
It is still another object for the provision of a header for effecting increased turbulent flow of the ambient water flowing between and around the heat transfer tubes.
It is an additional object to provide an improved header for one-piece keel coolers which enables the anode for such keel coolers to be less likely to strike debris and underwater objects.
Another object is the provision of a keel cooler having a smaller, and more streamlined profile to reduce drag as the vessel with the keel cooler moves through the ambient water.
Another object is to provide a header for a one-piece heat exchanger which provides for enhanced heat exchange between the coolant and the ambient cooling medium such as water.
Yet a further object is to provide a header for a one-piece heat exchanger which provides for more uniform flow of coolant through all tubes of the keel cooler, to improve the heat transfer as compared to equivalent, current headers.
A general object of the present invention is to provide a one-piece heat exchanger and headers thereof which is efficient and effective in manufacture and use.
Other objects will become apparent from the description to follow and from the appended claims.
The invention to which this application is directed is a one-piece heat exchanger, i.e. heat exchangers having two headers which are integral with coolant flow tubes. It is particularly applicable to heat exchangers used on marine vessels as discussed earlier, which in that context are also called keel coolers. However, heat exchangers according to the present invention can also be used for cooling heat generating sources (or heating cool or cold fluid) in other situations such as industrial and scientific equipment, and therefore the term heat exchangers covers the broader description of the product discussed herein. The heat exchanger includes two headers, and one or more coolant flow tubes integral with the header.